1. Field of the Invention
The invention relates to pressure swing adsorption for the separation of gases. More particularly it relates to the recovery and utilization of waste heat in pressure swing adsorption operations.
2. Description of the Prior Art
Pressure swing adsorption (PSA) processes are a desirable means for the separation and purification of gases, as in the production of oxygen or nitrogen by air separation. PSA processes involved the (1) selective adsorption of a more readily adsorbable component of a feed gas mixture at an upper adsorption pressure, with discharge of the less readily adsorbable component; (2) desorption of the more readily adsorbable component at a lower desorption pressure; and (3) repressurization from the lower desorption pressure to the upper adsorption pressure. Such operations, and variations thereof, are carried out in PSA systems comprising one or more adsorbent beds containing adsorbent material capable of selectivity adsorbing the more readily adsorbable component of a feed gas mixture from a less readily adsorbable component thereof. The processing is carried out in each bed on a cyclic basis interrelated to the carrying out of the processing sequence in each of the other beds in the system. A variety of commercially available adsorbent materials are suitable for use in PSA operations. A convenient class of adsorbent materials for such purposes is zeolitic molecular sieve materials, such as zeolite 5A or 13X materials, which are capable of selectively adsorbing nitrogen from feed air.
In the operation of PSA systems, heat is liberated upon adsorption, and heat is taken up by the adsorbent material upon desorption. Hence, the temperature of the adsorbent beds tends to rise during the adsorption step, and to drop during the desorption step. In typical PSA processing, such as for the production of oxygen and/or nitrogen from air, the forward flow of gas during adsorption exceeds the backward flow of gas during desorption. As a result, there is a net flow forward of heat, which tends to reduce the average temperature of the adsorbent beds employed in a PSA system.
PSA processes, particularly those using advanced adsorbents that are strongly adsorbent with respect to the more selectively adsorbable component of the feed gas, such as LiX, CaX or other zeolites prepared by ion-exchange of sodium zeolites, are very sensitive to adsorbent temperature. PSA processes, including vacuum pressure swing adsorption (VPSA) processes in which a subatmospheric desorption pressure are employed, appear to operate most favorably with a particular temperature within the adsorbent beds. Field data suggests that performance variations in excess of 10% would be likely if control of this optimal temperature is not addressed. It is important to note that VPSA systems employing advanced adsorbents utilize low pressure ratios relative to traditional PSA systems. As a result, there is a relatively small amount of heat being generated by the feed air machine, i.e. compressor, due to the heat of compression. This results in a feed air temperature, and resulting adsorbent temperature, that is largely a function of ambient conditions. Since advanced adsorbents sometimes require certain temperatures for desired performance, it is often important to find an efficient way to add heat to the adsorbent other than that provided by the feed air machine.
Due to the dependence of feed temperature on changes in ambient temperature, enough heat is not always generated through the feed air heat of compression to reach the desired adsorbent bed temperature levels. Adequate adsorbent bed temperature levels occur naturally under warm ambient conditions as the heat of compression from the feed air machine will generate enough heat in the feed stream to adequately warm the adsorbent above the ambient temperature to desired levels. In cases where the heat of compression is inadequate to serve such heat generation purposes, however, other means of heating the adsorbent, or the feed air or other feed gas, must be obtained in order to raise the adsorbent temperature and optimize system performance. In this regard, it should be noted that warm and cold ambient temperature conditions are not related to specific temperatures. Warm conditions exist, for purposes hereof, when the heat of compression from the feed air generates adequate heat to achieve optimal adsorbent bed temperatures, while cold conditions exist when some other means of adding heat to the VPSA or other PSA system is needed in order to obtain optimum process temperatures.
Prior attempts have been made to provide heat to an adsorbent bed that is operating at cooler than optimal conditions. While the heat of compression of the feed air is a desirable source of heat, no satisfactory augmentation of this generated heat has been adapted. A convective heat exchanger at the inlet of the adsorbent bed has been proposed, which also serves to provide developed heat flow distribution. It has also been proposed to use warm process gases to provide direct heat transfer to the adsorbent bed.
There remains, however, a need in the art for obtaining improved means for controlling adsorbent temperature to obtain enhanced PSA/VPSA performance. In particular, there is a need for a means to control adsorbent temperature while enhancing the energy efficiency of the overall system.
It is an object of the invention, therefore, to provide an improved process and system for controlling the temperature of adsorbent beds in PSA/VPSA air or other gas separation operations.
It is another object of the invention to provide an improved process and system for enhancing the energy efficiency of PSA/VPSA systems operating at desired adsorbent bed temperature conditions.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.